Metal whiskers having an essentially constant diameter of not more than 1000 angstroms



ay 5, 1964 F. E. LUBORSKY ETAL 3,132,022

METAL WHISKERS HAVING AN ESSENTIALLY CONSTANT DIAMETER 0F NOT MORE THAN1000 ANGSTROMS Filed June 29, 1961 mpoa w Qm mu m smu 2.53

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Diameter, Angst/ems s m .s m w Fig. 2

lnvenfars Fred E. Lubors/ry; Char/es R More/00k by M j The/r Afforne y3,132,022 METAL WHISKERS HAVING AN ESSENTIALLY CONSTANT DIAMETER OF NOTMORE THAN 1000 ANGtiTROlViS Fred E. Luborsky, Schenectady, and CharlesR. Morelock,

Ballston Spa, N.Y., assignors to General Electric Company, a corporationofNew York Filed June 29, 1961, Ser. No. 120,648 2 Claims. (Cl. 75.5)

This invention relates to magnets and more particularly to permanentmagnets constructed of nearly perfect single crystal whiskers.

The search for increasingly strong magnets and magnetic materials has,among other things, resulted in the use of single domain magneticparticles in the production of magnets having high coercive forces.Currently, the best single domain particles are produced byelectrodeposition from mercury baths and while their magnetic propertiesare superior to any previously produced, some reduction of maximummagnetic properties were felt to occur due to the extremely irregularshapes of these particles.

It is a principal object of this invention to provide ferromagneticwhiskers having improved magnetic characteristics.

Another object of this invention is to provide an improved magnetcomposed of a plurality of ferromagnetic whiskers.

Other objects and advantages of this invention will be in part obviousand in part explained by reference to the accompanying specification anddrawings.

In the drawings:

FIG. 1 is a graph showing the intrinsic coercive force of various typesof small ferromagnetic particles as a function of the diameters of theparticles; and

FIG. 2 is a greatly enlarged view showing the ferromagnetic whiskers ofthis invention.

The elongated ferromagnetic whiskers of this invention may be composedof iron, nickel, cobalt and alloys of these three metals. Probably themost important single characteristic which the whiskers must have toobtain higher coercive forces than heretofore is an essentially constantdiameter throughout the entire length of each whisker. Thus, thewhiskers are elongated, smooth and of substantially constant diameterthroughout their entire lengths. Magnets can be produced from thewhiskers by assembling a plurality of them together into one integralmagnet body by conventional techniques. I

Referring to the graph shown in FIG. 1 of the drawings, the intrinsiccoercive force of the whiskers of this invention is compared to theintrinsic coercive forces of the best small particles currentlyavailable. Curve illustrates the intrinsic coercive force, in oersteds,of generally spherically-shaped iron particles produced byelectrodeposition into mercury. Curve 11 indicates similar propertiesfor elongated magnetic particles also produced by deposition intomercury. A comparison of the values obtained from the two types ofparticles clearly indicates that the elongated configuration is muchmore effective in delivering high coercive forces than is the generallyspherical geometry. The coercive forces shown by curve 11 for themercury-deposited, elongated particles were obtained from specimenshaving the particles aligned with the long axis parallel to thedirection of magnetic measurement, although substantially the samecoercive forces are obtained from randomly distributed particles.

Curve 12 of the drawings indicates the intrinsic coercive forces of ironwhiskers produced according to the present invention. The valvesindicated by curve 12 were ob tained from specimens composed ofelongated iron 3,132,022 Patented M-ay'5, 1964 ice whiskers whose longaxes were randomly distributed with "respectto the direction of magneticmeasurement. Additional measurements were made on specimens composed ofelongated whiskers whose long axes were aligned generally parallel tothe directionof magnetic measurement and it-was found that the coerciveforces were substantially the same as the non-aligned Whiskers when thewhisker diameters were on the order of about 500 A. At larger diameters,for example at 1500 A., the coercive force of an aligned whiskerspecimen was about 37.5 percent less than that of the non-alignedspecimen. Theory indicates that the coercive force of aligned whiskersshould become larger than the coercive force of randomly distributedwhiskers when the diameter becomes'less than a critical diameter. Thetheoretical critical diameter is felt to be about 250 A., but test dataindicates that the actual critical diameter is somewhat larger, forexample between 400 and 500 A.

It is obvious, comparing curves 10-12, that for particles of similardiameters, those delivering the highest coercive forces are the whiskersof the present invention. Table I sets forth the magnetic properties ofseveral whiskers produced in accordance with this invention.

These particles were stable in air due to the presence of an oxidecoating on their surface which was only a few atoms thick. Because onlya small amount of oxide is present, there is no detrimental effect bythe oxide on the magnetic properties of the whiskers. It will be notedthat the coercive force increases significantly as the diameter of thewhiskers decreases, so that, for example, a

whisker having a diameter of about 250 A. has an intrinsic coerciveforce of about 3000 oerstedsa-t room temperature, this value beingappreciably higher than has previously been obtainable in other smallparticles.

Referring to FIG. 2 of the drawings, the general shape of the whiskersis indicated. It will be noted that they are elongated, andsubstantially uniform in diameter throughout the entire length thereof.It is the small diameter and the uniformity of this diameter which makesit possible for the present whiskers to attain their high magneticproperties.

The whiskers of this invention may be produced by placing a source metalbody and a substrate body in a closed chamber which is continuouslyevacuated. The chamber is then heated by any suitable means to atemperature such that the source metal body begins to evaporate. Thevapor then resulting will be deposited upon the substrate body, which ismaintained at a temperature sufficiently below that of the source metalbody to provide for deposition and growth of whiskers from its surface.A complete description of the method for producing whiskers can be foundin the copending application of Charles R. Morelock Serial No. 120,560,now abandoned, filed concurrently herewith and assigned to the sameassignee as the present invention.

The process described in the patent application just referred to can beetfectively used on the magnetic materials described in thisapplication, specifically iron, nickel, cobalt, and alloys of thesemetals, so that whiskers 3 of all of these materials can be obtained.For example, iron-cobalt whiskers containing 40 percent cobalt will havethe saturation induction increased by about 15 percent with acorresponding increase in coercive force since the coercive force, dueto shape anisotropy, is directly proportional to saturation induction.Thus, the maximum energy product will increase by about 32 percent.

Magnets can be produced using the present whiskers by agglomerating aplurality of the whiskers in a suitable matrix material. For example, ametal such as lead or a plastic matrix, may be used to combine thewhiskers. Measurements were made of a magnet produced by combining aplurality of whiskers on an insulating plastic film and it was found tohave coercive forces superior to mag nets previously produced. fromexisting small ferromagnetic particles.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. As an article of manufacture, an elongated whisker of a metalselected from the group consisting of iron, nickel, cobalt and theiralloys with each other, said whisker having an essentially constantdiameter of not more than about 1000 A, throughout the length thereof.2. An article as defined in claim 1 wherein the diameter does not exceed500 A. I

References Cited in the file of this patent UNITED STATES PATENTS Paineet al Mar. 7, 1961 OTHER REFERENCES

1. AS AN ARTICLE OF MANUFACTURE, AN ELONGATED WHISKER OF A METALSELECTED FROM THE GROUP CONSISTING OF IRON, NICKEL, COBALT AND THEIRALLOYS WITH EACH OTHER, SAID WHISKER HAVING AN ESSENTIALLY CONSTANTDIAMETER OF NOT MORE THAN ABOUT 1000 A. THROUGHOUT THE LENGTH THEREOF.